US 7876102 B2
A mixed mode tool uses an inductive source and detects galvanic currents and/or potentials at electrodes in proximity to a borehole wall to produce a resistivity image of the earth formation. Alternative, the magnetic field produced by a galvanic current is detected by an antenna coil and used to produce a resistivity image.
1. An apparatus for evaluating an earth formation, the apparatus comprising:
at least two electrodes configured to convey a current into the formation; and
at least one antenna configured to be conveyed in a borehole and provide a signal responsive to the current, the at least one antenna being in proximity to the formation, the signal being indicative of a resistivity property of the earth formation;
wherein the at least one antenna is positioned between a first one of the at least two electrodes and a second one of the at least two electrodes.
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10. A method of evaluating an earth formation, the method comprising:
conveying a current into the formation using at least two electrodes; and
using at least one antenna positioned between a first one of the at least two electrodes and a second one of the at least two electrodes for providing a signal responsive to the current, the at least one antenna being in proximity to the formation, the signal being indicative of a resistivity property of the earth formation.
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19. A computer readable medium product having stored thereon instructions that when read by a processor cause the processor to execute a method, the method comprising: for use with an apparatus for evaluating an earth formation, the apparatus comprising:
using a signal from an antenna positioned between a first electrode and a second electrode responsive to a current flow in an earth formation conveyed by the first electrode and the second electrode for producing an image of a resistivity property of the earth formation.
20. The medium of
This application is a continuation-in-part of U.S. patent application Ser. No. 11/521,016 filed on 14 Sep. 2006.
1. Field of the Disclosure
This disclosure generally relates to exploration for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this disclosure relates to highly localized borehole investigations employing the introduction and measuring of individual survey currents injected into the wall of a borehole
2. Background of the Art
Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a measure electrode (current source or sink) are used in conjunction with a diffuse return electrode (such as the tool body). A measure current flows in a circuit that connects a current source to the measure electrode, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure is a hybrid of the two.
There are several modes of operation of prior art devices: in one, the current at the measuring electrode is maintained constant and a voltage is measured while in the second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.
Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Patent No. 685727 to Mann et al., U.S. Pat. No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al. and U.S. Pat. No. 6,714,014 to Evans et al, each of which provide additional background information to this disclosure.
In the prior art devices, current is actively focused in the direction perpendicular to the borehole wall. There is a technical challenge to provide stable focusing conditions during the logging if the borehole walls are rough or the mud is very conductive. As soon as the focusing conditions are not met, the measurements are responsive to a considerable extent to the properties of the mud. The prior art devices do not specifically address the problems due to irregularities in the wall surface of the wellbore. If the wall of the wellbore is irregular, the measuring current path becomes distorted and the relationship between measured impedance and earth formation resistivity changed as result.
One embodiment of the disclosure is an apparatus for evaluating an earth formation. The apparatus includes at least two electrodes configured to convey a current into the formation and at least one antenna configured to be conveyed into a borehole and providing a signal responsive to the current, the at least one antenna being in proximity to formation and the signal being indicative of a resistivity property of the formation. The at least one antenna may be mounted on a pad extendable from a body of the logging tool. The at least two electrodes may be positioned on a mandrel of the logging tool. The at least two electrodes may be positioned on a pad extendable from a body of the logging tool. The two electrodes are configured to provide a current in a direction that is substantially parallel to a longitudinal axis of the logging tool or substantially orthogonal to a longitudinal axis of the logging tool. The apparatus may further include a processor configured to provide an image of the resistivity property of the earth formation. The apparatus may include an additional pad extendable from the boarding of the logging tool, the additional pad including at least one additional antenna. The at least one antenna is configured to be carried on a downhole assembly, the downhole assembly being selected from a bottomhole assembly and a logging string. The apparatus may further include a conveyance device configured to convey the at least one antenna into the borehole, the conveyance device being a wireline, a drilling tubular or a slickline.
Another embodiment of the disclosure is a method of evaluating an earth formation. The method includes conveying a current into the formation using two electrodes, and using at least one antenna conveyed in a borehole for providing a signal responsive to current, the at least one antenna being in proximity to the formation, and the signal being indicative of a resistivity property of the earth formation. The method may further include disposing the at least one antenna on a pad extendable from a body of a logging tool. The method may further include disposing the at least two electrodes on a mandrel of a logging tool. The method may further include disposing the at least two electrodes on a pad extendable from a body of a logging tool. The method may further include flowing the current in a direction that is substantially parallel to a longitudinal axis of the logging tool or substantially orthogonal to a longitudinal axis of the logging tool. The method may further include providing an image of the resistivity property of the earth formation. The determined resistivity property may be used for reservoir navigation, reservoir evaluation and/or reservoir development. The antenna may be conveyed on a bottomhole assembly or on a logging string. The antenna may be conveyed into the borehole using a wireline, a drilling tubular, or a slickline.
Another embodiment of the disclosure is a computer readable medium for use with an apparatus for evaluating an earth formation. The apparatus includes at least two electrodes configured to convey a current into the formation, and at least one antenna configured to be conveyed in a borehole and provide a signal responsive to the current, the at least one antenna being in proximity to the formation and the signal being indicative of a resistivity property of the formation. The medium includes instructions which enable a processor to use the signal to produce an image of the resistivity property of the earth formation. The medium may include a RAM, a ROM, an EPROM, an EAROM, a flash memory, and/or an optical disk.
The present disclosure is best understood with reference to the accompanying figures in which like numerals refer to like elements and in which:
This tool may be referred to as a “mixed mode” tool in that an inductive source is used and galvanic currents are detected by the electrodes. Specifically, a plurality of long transversal rectangular coils with the magnetic moment perpendicular to the axis of the borehole are used. Each transmitter loop is centered in the borehole and electrode pairs are placed on the pad attached to the borehole wall. This is a generic design and further variants are identified below. In a practical design each transmitted coil serves two pads with a number of electrode pairs on each pad. Each transmitter coil may have its own operating frequency to avoid the interference of the neighboring induction coils. By using an induction transmitter, an electric current can be injected into the formation.
At a low frequency and relatively close to the induction loop, the electric field does not depend on the conductivity of the formation and can be increased simply by increasing the operating frequency ω. In the case of a galvanic injection and non-conductive mud the injection current must go through quite a large capacitive résistance. This can be better understood from the simplified schematics in
If only electric field is measured, the measurements will be very sensitive to a relative variation of resistivity in the adjacent formation. To derive the absolute resistivity of the formation, some additional induction measurements and their combination with the galvanic readings are helpful.
The response of the tool design of
In a second model shown in
In the modeling, a 0.914-m long transmitter with a width of 0.1524 m was used. The operating frequency was 100 kHz. In the case of lower or higher frequencies (up to several MHz), the response can be approximately derived simply by linear resealing of the signal corresponding to 100 kHz frequency. A transmitter loop is placed in the nonconductive borehole environment with the radius of the borehole 10.795 cm. An electrode spacing of 0.25 inches or 0.5 inches (0.63 cm and 1.27 cm) was used to measure a potential drop Uz in the vertical direction parallel to the borehole axis.
The typical behavior of the electrical signal to the model is presented in
The dynamic range, which is the ratio between the maximum and minimum reading along the logging depth, is changing between 5 and 6 considering layers 1 in. and larger. We define a Normalized Dynamic Range (NDR) as a ratio of a signal dynamic range to a resistivity contrast of the corresponding media. In the model of
Next, examples showing the influence of the distance between the receiver electrodes and the borehole wall are presented. The results of mathematical modeling for the same benchmark model of
Turning next to
The embodiments of the disclosure discussed above used button electrodes for measuring a voltage difference resulting from currents induced in the formation by an antenna. Other embodiments of the disclosure use the principle of reciprocity to modify the designs discussed above. In these other embodiments, a current is conveyed into the formation using an electrode and an inductive sensor is used to measure the magnetic field in the formation due to the current flow. This magnetic field is related to the formation conductivity.
An example is shown in
Another embodiment is shown in
Yet another embodiment is shown in
It should be noted that in the embodiments discussed in
It should further be noted that if the medium is anisotropic, the configuration of
The processed resistivity measurements may be recorded on a suitable medium. In addition, the resistivity measurements may be used in reservoir evaluation. Resistivity images made with wireline instruments are useful in determining formation dips, reservoir boundaries and fault directions. All of these provide useful insight in establishing recoverable reserves and productivity of reservoirs. This includes determination of completion operations, setting of casing, fracturing operations and the like. In addition, measurements made in an MWD setting a useful in reservoir navigation wherein the directional drilling may be controlled based upon a resistivity image.
The processing of the data may be done with the use of a computer program implemented on a suitable machine readable medium that enables the processor to perform the control and processing. The term processor as used in this application is used in its traditionally-broad sense and is intended to include such devices as single-core computers, multiple-core computers, distributed computing systems, field programmable gate arrays (FPGAs) and the like. The machine readable medium referenced in this disclosure is any medium that may be read by a machine and may include magnetic media, RAM, ROM, EPROM, EAROM, flash memory and optical disks. The processing may be done downhole or at the surface. In an alternative embodiment, part of the processing may be done downhole with the remainder conducted at the surface.
The disclosure has been described with reference to a wireline conveyed logging tool. The principles discussed above may also be used in a measurement-while-drilling (MWD) implementation in which the logging tool is part of a bottomhole assembly (BHA) conveyed on a drilling tubular. The method may also be used with the logging tool conveyed on a slickline. For the purposes of the present disclosure, the term “downhole assembly” may be used to describe a BHA as well as configurations in which the logging tool is part of an assembly conveyed on a wireline or slickline.
The following definitions are helpful in understanding the present disclosure.
While the foregoing disclosure is directed to the preferred embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.